RESPIRATION 



47 



P/eura 



bing of the surfaces due to the movement of the lungs 

 and chest in breathing. The liquid serves as a natu- 

 ral oil to lubricate 

 the membranes and 

 hence to make 

 breathing easy. 



50. How the Air 

 gets into the Lungs. 

 We have learned 

 about the passages 

 through which the 

 air is conveyed to 

 the lungs. But how 

 do we breathe ? 

 What makes the air 

 go into these pas- 

 sages ? We shall 

 try to answer these 

 questions. We know that the chest rises and falls and 

 the ribs move in and out in breathing. We cannot see 

 the diaphragm, but that dome-shaped partition moves 

 up and down when we breathe. The chest enlarges in 

 all directions, due to the contraction of the muscles of 

 the neck (scalene) and the muscles of the ribs (inter- 

 costal) and the diaphragm. All these muscles work 

 together in such a way as to reenforce each other con- 

 stantly. When these muscles contract, the chest en- 

 larges, and the lungs also enlarge to fill the chest 

 completely. The amount of these movements we can 

 easily measure with a tapeline. Try it and see how 



FIG. 20. Showing how the pleurae cover the 

 lungs (Zuppke). 



